1. Field of the Invention
This invention generally relates to current switching amplifier inverters and, more particularly, to an improved dual current source MOSFET turn-on/off circuit for use in such inverters.
2. Description of the Prior Art
The P-channel MOSFET (metal oxide semiconductor field effect transistor) with its high gate threshold voltage (on the order of 2 to 4 volts), fast turn on/off times and polarity makes an ideal component for pulse width modulated switching regulators. A typical application is shown in FIG. 1A. In this circuit, the P-channel MOSFET Q.sub.1 acts as a high-speed switch to modulate the voltage appearing on the catch diode CR.sub.1 and inductor L.sub.1. This voltage is either V.sub.in or zero. Closing switch SW.sub.1 turns on N-channel MOSFET Q.sub.2 which, in turn, turns on Q.sub.1. The output voltage, V.sub.o, is equal to the "on" time, T.sub.1, divided by the total period, T.sub.t, times the input voltage (neglecting losses), as shown in FIG. 1B.
This circuit, while having the very nice advantage of not requiring a bias supply at the +V.sub.in potential to control Q.sub.1, has three undesirable features: First, power dissipation increases as V.sub.in increases. Second, turn-on time decreases as V.sub.in increases, producing a compounding effect on circuit noise increases caused by faster rise times and larger voltage excursions. Third, V.sub.in must exceed a large threshold to get any output.
A bias supply and other circuitry at the +V.sub.in -to-zero potential to control Q.sub.1 is required if an N-channel MOSFET is used as shown in the circuit of FIG. 2. Obvious design complications aside, this configuration greatly increases the circuit noise because point P (and the entire turn-on/off circuit) swings up and down between O and +V.sub.in at the chopping rate.